An invisible donut of trapped, hot particles surrounding Saturn is all bent out of shape — a finding that astronomers can't yet explain.

A similar "ring current" phenomenon occurs around Earth as a relatively stable donut when present, but new Cassini spacecraft images show Saturn's loop is a lopsided mess.

"It's curious that Saturn's ring current isn't symmetric," said Don Mitchell, an astrophysicist at Johns Hopkins University who helped examine the images beamed back to Earth. "We think the solar wind is squishing the sunward side of the ring current, kind of like a wind sock."

Planets with magnetic fields can trap hot particles within their clutches to form giant electrified clouds-the ring currents-that are invisible to the naked eye.

Earth's current ring, shown in this image peering down on the north pole, doesn't rotate with the planet, but remains fixed relative to the sun. The white lines show the Earth's position and orientation.

largely of oxygen and is always present. The saturnian moon Enceladus is responsible for the electric halo, as it consistently spews water vapor from its depths to feed the ring current with oxygen and hydrogen ions.

Because oxygen is far heavier than hydrogen, Mitchell said, Saturn's ring current can distort the planet's magnetic field and make for an odd shape.

"The heavier oxygen is like a rock on a string, stretching the magnetic field of Saturn," Mitchell said.

More mysterious to Mitchell and his colleagues, however, is a "clump" of electrified particles within the ring that rotates in sync with the planet roughly every 10 hours and 47 minutes.

Cassini's images show the bright clump orbits Saturn between 300,000 and 634,000 miles (485,000 and 1,000,000 kilometers) away from the planet's surface, but astronomers have not yet figured out what creates it nor why it moves so quickly.

"Saturn is a big fast rotator. The clump seems loosely hooked to the planet, yet rotates with it," Mitchell said. "It may be connected with Saturn's ring current, but we just don't know. This is something we're working very hard to figure out."

Stamatios Krimigis, also an astrophsycist at Johns Hopkins who examined the images, is presenting them Thursday at the European Planetary Science Congress in Potsdam, Germany.